<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style type="text/css">
        * {
            margin: 0;
            padding: 0;
        }

        #container {
            display: block;
            /* display: flow-root; */
            width: 300px;
            margin: 50px;
            background: yellow;
            border: 1px solid red;
        }

        #item-1 {
            width: 50px;
            height: 400px;
            background: orange;
            text-align: left;
            float: left;
        }

        #item-2 {
            width: 100px;
            height: 50px;
            background: greenyellow;
            text-align: left;
        }

        #item-3 {
            width: 100px;
            height: 50px;
            background: dodgerblue;
            text-align: right;
        }
    </style>
</head>

<body>
    <div id="container">
        <div id="item-1">A</div>
        <div id="item-2">B</div>
        <div id="item-3">C</div>
    </div>
</body>


</html>